Road Type and Urbanization Effects on Roadside Plant Diversity along a Rural‒Urban Gradient in Welmara District, Oromia, Ethiopia

Road type and urbanization create unique environmental settings that strongly influence roadside vegetation diversity. However, the synergistic effects of road type, urban intensity, and distance from road edges remain insufficiently elucidated, particularly in rapidly urbanizing areas such as the central highlands of Ethiopia. Therefore, this study aimed to investigate how road type and urbanization affect both aboveground plants and the soil seed bank along a rural‒urban gradient in Welmara District, Oromia, Ethiopia. Plant species data were collected from 72 quadrats across twelve sites, and asphalt and gravel roads were compared at three distances (0–10 m, 10–20 m, and 20–30 m) within urban and rural locations. General linear models (GLMs), simple linear regressions, permutational multivariate analysis of variance (PERMANOVA), indicator species analysis (ISA) and nonmetric multidimensional scaling (NMDS) were used for data analysis. A total of 139 aboveground species were identified (78.42% native, 15.83% exotic, and 5.75% unidentified), whereas the soil seed bank contained 72 species (80.60% native, 19.40% exotic). The results suggested that rural areas supported greater species richness than did urban areas. The percentage of exotic species is significantly greater in areas adjacent to asphalt roads (80%) and within urban landscapes (70%) than in those near gravel roads (25%) and rural regions (30%). A highly significant negative linear relationship was observed between the percentage of exotic cover and distance from the road edge across both locations and road types (p < 0.001). Conversely, the soil seed bank did not significantly differ across road types or urbanization levels (p > 0.05). These findings emphasize that urbanization, road type, and proximity to roads are critical causes of exotic species distribution in aboveground plants. Moreover, it highlights the importance of integrating both aboveground and soil seed bank perspectives for effective restoration strategies through the promotion of native plant communities in the study area.